In the field of the treatment of ribbon-shaped paper material, in particular in the field of the treatment of veils of paper coupled with each other by means of a lamination operation and the application of an adhesive, rolls are envisaged which must make the processing operations uniform over the whole surface of the paper which advances longitudinally sliding on the roll.
For this purpose, current rolls generally consist of a single cylindrical body equipped with side neck-holes, on which the respective bearings are housed, which follow the rotation around the axis and the bending of the body caused by the coupling pressure. This arrangement has a serious drawback: the pressure between the coupled rolls causes them to bend and consequently the paper passing between them is not subjected to a uniform treatment and does not satisfy the high-quality requirements requested by the market.
In the field of the embossing, for example, it is known that two or more veils of paper previously embossed are joined by the application of a glue and lamination of the veils between counter-rotating rolls to obtain the final product. The embossing operation is effected by passing each veil, before being glued to the other veil, between two rolls, one rubber, the other steel incised on the surface. The incised roll, or otherwise generated, is equipped with tips or protuberances and is used for embossing the single veil of paper coupled with the rubber roll.
In these pairs of rolls, which can be called embossing units, there are the above drawbacks caused by the deformation of the rolls through which the veils to be embossed pass.
As already mentioned, in fact, in the pressurized coupling between a steel roll and a rubber roll, during the operative functioning, there is a tendency towards a bending of the rolls. As a result, the steel roll, having surface protuberances, does not correctly and uniformly incise the whole transversal development of the veil of paper. This leads to a non-uniform embossing in the transversal development, which is not adequate for the high-quality standards required by the market, specifically due to this lack of constant characteristics.
A first proposal envisages the use of rounded rubber rolls, whose rounding however is only ideal for an extremely restricted range of pressure values, outside which the positive effects of the rounded rolls is nullified. The consequent necessity of using special rolls for the various ranges, makes the solution quite impracticable due to the costs of the various rolls and costs and times necessary for the assembly and dismantling.
A further solution currently applied comprises reducing the pressure exerted by the rubber roll, obtaining however a less evident embossing, i.e. less marked, thus providing an end-product with a limited thickness, which does not meet market requirements.
An alternative solution adopts complicated mechanical arrangements, called skewing or with rolls having oblique axes, in an attempt to uniform the incision of the paper. This solution allows a decrease in the embossing value of the rubber roll. These arrangements are complex from a structural point of view of the machine and also for the user for their correct functional set-up with difficulty in aligning the paper unwound. There is also a significant production cost and a precocious and non-uniform wear of the rubber roll.
Another proposal consists in envisaging a particular arrangement which attempts to overcome the drawback of the bending of pressurized coupled rolls, as suggested in international patent application WO-A-2009/010999.
In this solution, the rolls are transformed from single rotating cylindrical bodies to a structure or arrangement consisting of a fixed central shaft (the neck-holes are wedge-inserted in flanks/arms) on which two groups of bearings of the oscillating type are assembled, at a distance shorter than the table.
This arrangement allows the rotation, around the longitudinal axis of the fixed central shaft, of a jacket that forms the surface facing the outside of the roll. This configuration in fact relieves a great deal of the bending, due to the coupling pressure between the rolls, on the fixed central shaft, thus allowing the outer jacket to remain almost parallel or with a negligible rise.
It should be pointed out that this arrangement has certain assembly difficulties due to the wedge-inserted central shaft. The bearings in this arrangement undergo excessive heating partly due to their orientable nature and stress in play. The production of this particular arrangement also requires a specific projecting, which is complex and costly, of both the rolls themselves and also the whole machine suitable for receiving the latter.
It has also been observed that the same problem is present in the rolling presses or pairs of rolls used for levelling the thickness of the paper and, more generally, in production processes which envisage the compression of a strip of paper material passing between two entrainment rolls.